1. Field of the Invention
This invention relates generally to switching regulators, and more particularly to circuits designed to filter the ripple component of the regulator's output voltage.
2. Description of the Related Art
The operation of a switching regulator requires that one or more switching elements be switched on and off at a particular switching frequency, which causes the current in the regulator's storage inductor to increase and decrease at the switching frequency. This increasing and decreasing inductor current is largely filtered by the regulator's storage capacitor, but some of this oscillation inevitably appears as a ripple component in the regulator's output voltage.
This ripple component can be particularly problematic when of the switching regulator's output voltage is driving an analog load, as the ripple frequencies typically get through the amplifier, ADC, etc. and corrupt the output. The ripple is not broadband noise; rather, it is coherently related to the switching regulator clock and is harmonically rich as determined by the mark space ratio the regulator is running at.
As ripple is generally undesirable, many systems include circuitry designed to attenuate the ripple component. This usually takes the form of one or more low pass filters made from various manifestations of capacitors, inductors and/or ferrite beads; a linear post-regulator connected between the switching regulator's output and the load might also be used. The attenuation necessary is application-specific, typically depending on the tolerance of the load to ripple.
To achieve a desired level of filtering, it may be necessary to use several series stages. A single inductor and capacitor connected in a low pass configuration can provide 12 dB/octave attenuation; however, attenuation on the order of 30 dB is often necessary. Use of cascaded filter stages can provide additional attenuation, but typically necessitates intermediate buffering. A notch-type filter might also be considered; however, aligning the resonant notch with the fundamental frequency of the ripple component can be difficult.